IP-PBX                                         P-CSCF
    |                                               | 
    |----(1) REGISTER------------------------------>| 
    |        Security-Client: sdes-srtp;mediasec    |
    |        Proxy-Require: mediasec                |
    |        Require: mediasec                      |
    |                                               | 
    |<---(2) 401------------------------------------|
    |        Security-Server: msrp-tls;mediasec     | 
    |        Security-Server: sdes-srtp;mediasec    | 
    |        Security-Server: dtls-srtp;mediasec    |
    |                                               | 
    |----(3) REGISTER(with Authorization Header)--->| 
    |        Security-Client: sdes-srtp;mediasec    |
    |        Proxy-Require: mediasec                |
    |        Require: mediasec                      |
    |        Security-Verify: msrp-tls;mediasec     | 
    |        Security-Verify: sdes-srtp;mediasec    | 
    |        Security-Verify: dtls-srtp;mediasec    |
    |                                               | 
    |<---(4) 200------------------------------------|
    |                                               | 
    |                                               | 
    |----(5) INVITE-------------------------------->| 
    |        Security-Verify: msrp-tls;mediasec     | 
    |        Security-Verify: sdes-srtp;mediasec    | 
    |        Security-Verify: dtls-srtp;mediasec    |
    |        Proxy-Require: mediasec                |
    |        Require: mediasec                      |
    |        a=3ge2ae:requested                     |
    |        a=crypto:1 AES_CM_128_HMAC_SHA1_80 inline:EpcgtOdT5qd...
    |                                               | 
    |<---(6) TRYING---------------------------------|
    |                                               | 
    |<---(7) RINGING--------------------------------|
    |                                               | 
    |<---(8) 200------------------------------------|
    |        a=crypto:1 AES_CM_128_HMAC_SHA1_80 inline:lnfakjh2sd1...
    |                                               |